Empirically, 1 s is widely used as the sampling period in the digital measurement of sound level. In order to clarify the theoretical background of this optimum sampling period, we consider the correlation between the data of each sample and discuss the effectiveness of the data from statistical point of view. In the case of general road sound, the optimum of sampling period is the value of Δtfor which the autocorrelation coefficient ρ(Δt) of the wave form is equal to12, where the envelope wave form is rms value obtained by the integration circuit corresponding to the FAST dynamic characteristics, by weighting the sound withAcharacteristics. This value is a function of the distance between the lane and the measuring point on the road and the vehicle speed. We also measured various kinds of road sounds and obtained the autocorrelation of the time series for each kind. For present status of the road condition in Japan, the optimum of sampling period is in the order 0.5–1.0 s.